Self-reversing ribbon drive

ABSTRACT

A self-reversing drive mechanism for a ribbon or other web-like article is disclosed. The mechanism includes a pair of spools, spaced to serve alternately as supply and take-up spools, and a driver gear shiftable in response to radial gear tooth forces thereon between positions wherein it is drivingly coupled to first one and then the other of the spools. The radial tooth forces are determined by tension in the ribbon, and reach the values necessary to effect shifting of the driver gear after the ribbon is fully unwound from the supply spool.

United States Patent Cavella 51 June 20, 1972 [54] SELF-REVERSING RIBBON DRIVE [72] Inventor: James J. Cavella, Somerdale, NJ.

[7 3] Assignee: Datadyne Corporation, King of Prussia,

[22] Filed: May 6, 1970 211 Appl. No.: 34,957

[52] U.S. CI ..242/67.4, 74/354, 74/405, 197/162, 197/164, 242/201 [51] Int. Cl. ..G1lbl5/44 [58] Field of Search ..242/54, 55, 200, 201, 202, 242/67.4; 74/354, 384, 404, 405, 406; 197/160,

[56] References Cited UNITED STATES PATENTS 3,405,565 10/1968 DeValliere ..74/354X 2,896,873 7/1959 Mageoch ..242/67.4

2,906,184 9/1959 Hennig ct al ..74/405 X 3,529,789 9/ l 970 Morgan ..242/201 FOREIGN PATENTS OR APPLICATIONS 1,088,603 10/ 1 967 Great Britain ..197/162 Primary Examiner-Stanley N. Gilreath Assistant Examiner-Wemer H. Schroeder Attorney-Seidel, Gondal & Goldhammer [57] ABSTRACT A self-reversing drive mechanism for a ribbon or other weblike article is disclosed. The mechanism includes a pair of spools, spaced to serve alternately as supply and take-up spools, and a driver gear shiftable in response to radial gear tooth forces thereon between positions wherein it is drivingly coupled to first one and then the other of the spools. The radial tooth forces are determined by tension in the ribbon, and reach the values necessary to effect shifiing of the driver gear after the ribbon is fully unwound from the supply spool.

14 Claims, 5 Drawing Figures PATENTEuJum m 3,670,981

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JAMES J. CAVELLA I 8) MM aw PATENTEnJum m2 SHEET 2 BF 2 mA Wu %E Z amwm SELF-REVERSING RIBBON DRIVE This invention relates to a self-reversing ribbon drive mechanism, and more particularly, to a drive mechanism for a ribbon or other web-like member wherein tension in the ribbon causes a constantly driven drive gear to shift between respective operative positions wherein it drives the ribbon in opposite directions.

In its broadest aspect, this invention relates to an apparatus for obtaining a self-reversing motion of a flexible member, such as a printing ribbon or the like. Although it is described herein as having particular applicability to ribbons for printers, the principles of the invention are applicable to apparatus for handling other flexible web-like members as well.

In printing devices using transfer ribbons, it is conventional to provide ribbon drive devices for advancing the ribbon after printing of one or more characters. Advancement of the ribbon greatly enhances the clarity of the printed characters. It is conventional to draw the ribbon from a supply spool to takeup spool until the supply spool is fully unwound. Then, the direction of ribbon feed is reversed, the original take-up spool becoming the supply spool, and vice versa. Periodic reversal of the direction of ribbon feed, it has been found, increases the useful life of the ribbon.

Heretofore, reversal of the ribbon drive has conventionally been accomplished by means of switches, relays, solenoids, clutches and a variety of mechanical linkages, many of which have proven to be costly, complicated and unreliable.

It has also been suggested, as in US. Pat Nos. 2,621,864 and 3,179,948 that gearing, shiftable in response to sensed web tension, be used to effect reversal. In the first of these, directed to a reversible winder for strip film, a complex planetary gear train, associated with a spring-urged movable motor mount, is employed. In the latter, wherein ribbon drive in intermittent rather than constant, worm gearing, rather than simple spur gearing is used.

It is a principal object of this invention to provide a selfreversing ribbon drive mechanism using ordinary spur gears having a conventional tooth profile.

It is another object of this invention to provide a simple and effective self-reversing ribbon drive, wherein shifting movement of a drive gear to effect reversal is positive and reliable, without need of expedients such as 'movable or spring-biased motor mounts.

It is yet another object of this invention to provide a selfreversing ribbon drive mechanism wherein constant rather than intermittent motor drive is accommodated.

. Other objects will appear hereinafter.

The foregoing and other objects are accomplished, in a presently preferred form of the invention, by apparatus wherein the first and second web-bearing spools are alternately driven in opposite directions by a driver gear, shiftable between respective positions wherein it is drivingly coupled to first one and then the other of the spools. Shifting of the driver gear out of driving engagement with one of the spools and into engagement with the other is initiated in response to the increase in radially directed tooth forces occasioned by exhaustion of the web from the spool functioning as the supply spool. The possibility of the driver gear lodging at a deadcenter position, wherein it is coupled to neither of the spools, is effectively eliminated in the present apparatus through the use of a rotating eccentric weight, the periodic impulses of which prevents the driver gear from coming to rest in any but its respective spool-driving positions. Mechanical biasing means, such as magnets or the like, are also associated with the driver gear to maintain it in its respective spool-engaging positions.

For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it is being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a side elevation view, partly in phantom, showing a ribbon drive mechanism in accordance with the present invention.

FIG. 2 is a cross-sectional view, taken along the line of 2-2 in FIG. 1.

FIG. 3 is a side elevation view of the mechanism showing the driver gear in a first position, drivingly coupled to one of the ribbon spools.

FIG. 4 is a side elevation view showing the driver gear in another position, drivingly coupled to the other of the spools.

FIG. 5 is a view, partly broken away, showing the interengagement of the driver gear and a gear associated with one of the spools, and illustrating the matter in. whichthe tooth forces thereon may be resolved.

Referring now to the drawings in detail, wherein like nus merals indicate like elements, there is seen-in FIG. 1 a selfreversing ribbon drive mechanism designated generally by the reference numeral 10. The ribbon drive mechanism 10 is shown associated with the frame 12 of a high speed printer, designated generally by the reference number 14, of the sort used in the data processing field. It should be understood, however, that the utility of the presentribbon drive mechanism is not limited to such an application.

Referring to FIG. 2, opposite ends of a flexible ribbon 16 are coupled to respective spools l8 and 20, received upon and rotatable with a pair of drive shafts 22 and 24. The drive shafts 22 and 24 are journaled in the frame 12, and in the illustrated embodiment, have parallel axes of rotation. Guide spindles 26- and 28, coupled to the frame 12, serve to guide and support the ribbon 16 adjacent the spools l8 and 20.

When it is desired to advance the ribbon 16 in one direction, the shaft 22 and associated spool 18 are driven ina counter-clockwise direction as seen in FIG. 1. In this instance, the spool 18 serves as a take-up spool, and the spool 20 as a supply spool. When it is desired to advance the ribbon. 16 in the other direction, the drive shaft 24 and its asociated spool 20 are driven in a clockwise direction as seen in FIG. 1. In this instance, it is spool 20 which serves as the take-up spool, and the spool 18 as the supply spool.

Rotation of the drive shafts 22 and 24 in their respective directions is effected by a uni-directional electric motor 30, seen in FIG. 1. The torque and rotation of the motor 30 are imparted to the drive shafts 22 and 24 by a gear train designated generally by the reference numeral 32, which will now be described in detail.

A cantilever arm 34, best seen in FIG. 1, is afiixed to the frame 12. Referring to FIG. 2, associated with one end of the cantilever arm 34 is a bearing portion 36, which provides an external cylindrical bearing surface 38 and an internal bearing sleeve 40. A shaft 42 is rotatably received in the bearing sleeve 40.

Coupled to opposite therewith are a drive pulley 44 and a pinion secures the pinion 46 to the shah 42.

Referring again to FIG. 1, a drive belt 50, threaded over the drive pulley 44 and another pulley 52 associated with the output shaft 54 of the motor 30, transmits rotation to the shaft 42 and pinion 46.

Rotatably mounted on the cylindrical bearing surface 38 is toggle arm 56. A shaft 58 is anchored at one of its ends to the toggle arm 56. A cluster gear 60, which includes coaxial gear portions 62 and 64, is rotatably mounted on the shaft 58. The gear portion 62 is in meshed and constantly driven relation with the pinion 46. The gear portion 64 is in meshed and constantly driving relation with another gear 66, affixed to a shaft 68 ends of the shaft 42 for rotau'on 46. A pin 48 The shaft 68, which in the illustrated embodiment is parallel to the abovementioned shaft 58 is journaled, by means, for example, of bearings 70 and 72, in the toggle arm 56. Secured to the shaft 68 and rotatable therewith is a disk 74, to which an eccentrically placed fly-weight 76 is affixed. The purpose of the disk 74 and fly-weight 76 will be described in detail later. Also secured to the shaft 68 and rotatable therewith is a driver gear or pinion 78.

As is apparent from FIGS. 1 to 4, a gear 80 is affixed to an end of the drive shaft 22 for the spool 18. Also, a gear 82 is affixed to an end of the drive shaft 24 for the spool 20.

Referring to FIGS. 1, 3and 4, an idler gear 84, rotatably mounted on a stub shaft 86 affixed to the frame 12, is provided. The idler gear 84 is in constant meshing interengagement with the gear 82. The idler gear 84 and the gear 80 associated with the drive shaft 22 lie in a common plane with the pinion 78.

The pinion 78 is shiftable between respective positions wherein it is drivingly coupled to the gear 80 and drive shaft 22, and the idler gear 84, gear 82 and drive shaft 24. In FIGS. 3 and 4, the pinion 78 is seen in the above-mentioned respective positions.

Shifting of the pinion 78 occurs upon angular rotation of the toggle arm 56 about the shaft 42. In FIG. 3, the toggle arm 56 is shown in its position wherein the pinion 78 meshes with an drivingly engages the gear 80.

In FIG. 4, the toggle arm 56 is shown in a position wherein the pinion 78 meshes with and drivingly engages the idler gear 84.

Means are provided in association with the toggle arm 56 to maintain the arm in one or the other of the positions shown in FIG. 3 and 4. For this purpose, a magnet 88 is affixed adjacent the distal upper end of the toggle arm 56. In the illustrated embodiment, the magnet 88 is retained between a pair of pole pieces 90 and 92, the pole piece 90 being affixed to the toggle arm 56 by a screw 94.

An armature 96, of magnetic material, provides a pair of parallel legs 98 and 100, extending outwardly away from the frame 12. As is seen in FIGS. 1, 3 and 4, the legs 98 and 100 straddle the magnet 88 and pole pieces 90 and 92, and contact the pole pieces 90 and 92 when the toggle arm 56 is at the respective positions shown in FIGS. 3 and 4.Magnetic attrac tion between the pole pieces 90 and 92 and the legs 98'and 100, together with the-force of gravity, tend to maintain the toggle arm 56 in the aforementioned positions, wherein the pinion 78 drivingly engages either the gear 80 or the idler gear 84.

Referring to FIGS. 3, 4 and 5, shifting. of the toggle arm 56 and pinion 78 in response to web tension will now be described. Referring first to FIG. 5, meshing of the pinion 78 and gear 80 is seen in an enlarged scale. As is well known, in involute gearing, force is transmitted between meshing gears along a line of action extending between the base circles of the respective gears, and intersecting a line between centers of the gears at an angle known as the working pressure angle 1). The tooth forces can be resolved into components F, and F,, extending in the radial and tangential directions with respect to the meshed gears. The effect of the radially directed forces, of course, is to tend to separate the meshed gears. This effect, in the present invention, has been used to trigger shifting of the pinion 78 between its positions illustrated in FIGS. 3 and 4.

The tooth forces sensed by the pinion 78 are a function of tension in the ribbon 16. Referring to FIG. 3, rotation of the pinion 78 results in a corresponding rotation of the gear 80 and its associated drive shaft 22 and spool 18. The spool associated with the drive shaft 24, gear 82, and idler gear 84, are free to rotate. Accordingly, as the pinion rotates in its usual clockwise direction, the gear 80, drive shaft 22 and spool 18 rotate in the counter-clockwise direction shown by the arrow in FIG. 3. Ribbon 16 is unwound from the spool 20, causing free rotation of the drive shaft 24 and gear 82 in the illustrated counter-clockwise direction. When ribbon 16 is completely unwound from the spool 20, rotation of that spool stops. Rotation of the pinion 78 and gear 80 continues, however, tightening the portion of the ribbon 16 already wound upon the spool 20. As the ribbon 16 is tightened on the spool 18, the torque transmitted from the pinion 78 to the gear 80 increases, and the radially directed force tending to separate the pinion 78 and the gear 80 increases as well.

The radially directed force F, manifests itself as a moment tending to cause rotation of the toggle arm 56 in a counterclockwise direction about the shaft 42. The moment created by F, is resisted by a moment due to magnetic attraction of the pole pieces 90 and 92 and the leg 100, and also by a much smaller moment attributable to the weight of the toggle arm 56 and the parts thereon. Eventually, however, F,., acting over its moment arm, becomes sufficiently strong to cause separation of the pole pieces 90 and 92 and the leg 100.

As is well known, the force of magnetic attraction varies inversely as the square of the distance between the attracted bodies. Hence, upon separation of the pole pieces 90, 92 and the armature 100, the attractive force due to the magnet 88 falls off rapidly. Moreover, the tightly wound ribbon acts as a spring to maintain torque on the pinion 78 until it completely disengages from the gear 80. The inertia of the toggle arm 56 and the various parts thereon tends to carry the arm 56 overcenter, so that the pole pieces 90 and 92 ultimately contact the armature 98, and the pinion 78 falls into driving engagement with the idler gear 84. Firm engagement of the pinion 78 and idler gear 84 is insured by the magnetic attraction between the pole pieces 90, 92 and the armature 98. The rotating weight 76 effectively eliminates the possibility of the toggle arm 56 and pinion 78 lodging at dead-center, in engagement with neither the gear 80 nor the idler gear 84.

Whether engaged with the gear80 or the idler gear 84, the pinion 78 is driven by the motor 30 in the same direction. The idler gear 84, however, causes the gear 82 to rotate in a clockwise direction, opposite from the direction of rotation of the gear 80. Thus, when the gear 82 is driven, the ribbon 16 is wound around the spool 20. Such winding continues until all of the ribbon 16 is exhausted from the spool 18, the ribbon tension builds sufficiently to cause reversal once again.

It should be apparent that if the ribbon 16 becomes tangled or disarrayed in such a way that the torque required to wind it becomes sufficiently great, the direction of winding will change automatically, to assist in unfouling the ribbon 16. Thus, damage to the ribbon 16 and the drive mechanism 10 is automatically adverted.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specifications as indicating the scope of the invention.

I claim:

1. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive the other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion shiftable between respective positions wherein it is drivingly coupled to one or the other said driven gears, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion to initiate shifting of said pinion, biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said gears, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces.

2. Apparatus in accordance with claim 1, wherein said biasing means comprises magnetic means.

3. Apparatus in accordance with claim 1, wherein said support member comprises an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shifting of said pinion.

4. Apparatus in accordance with claim 3, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positions of said arm member in which said pinion is drivingly coupled to one of said driven gears being disposed on opposite sides of the vertical position, and said rotating eccentric weight being effective to prevent lodging of said arm in intermediate positions wherein said pinion is out of engagement with both driven gears.

5. Apparatus in accordance with claim 4, wherein said biasing means comprises magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means suaddling said arm member and positioned to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.

6. Apparatus in accordance with claim 1, and means coupled to said pinion for driving said pinion unidirectionally, and an idler gear drivingly engaged with one of said driven gears, said pinion being engaged with said idler gear when drivingly coupled to one of said driven gears.

7. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion shiftable between respective positions wherein it is drivingly coupled to one or the other of said driven gears, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion to initate shifting of said pinion, means coupled to said pinion for driving said pinion unidirectionally, and an idler gear drivingly engaged with one of said driven gears, said pinion being drivingly engaged with said idler gear when drivingly coupled to said one of said driven gears, said support member comprising an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shift of said pinion.

8. Apparatus in accordance with claim 7, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positions of said arm member in which said pinion is drivingly coupled to one or the other of said driven gears being disposed on opposite sides of the vertical position.

9. Apparatus in accordance with claim 8, and biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said driven gears, said biasing means comprising magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means straddling said arm member and position to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.

10. Apparatus in accordance with claim 9, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces, said rotating eccentric weight means being driven by said means for driving said pinion.

11. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive the other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion, means for driving said pinion in a predetermined direction, said pinion being shiftable between respective positions wherein it is drivingly coupled to one or the other of said driven gears while said means drives said pinion in said direction, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion while said pinion rotates in said direction to initiate shifting of said pinion, said support member comprising an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shifting of said inion.

l2. pparatus in accordance with claim 11, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positions of said arm member in which said pinion is drivingly coupled to one or the other of said driven gears being disposed on opposite sides of the vertical position.

13. Apparatus in accordance with claim 12, and biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said driven gears, said biasing means comprising magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means straddling said arm member and position to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.

14. Apparatus in accordance with claim 13, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces, said rotating eccentric weight means being driven by said means for driving said pinion. 

1. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive the other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion shiftable between respective positions wherein it is drivingly coupled to one or the other said driven gears, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion to initiate shifting of said pinion, biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said gears, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces.
 2. Apparatus in accordance with claim 1, wherein said biasing means comprises magnetic means.
 3. Apparatus in accordance with claim 1, wherein said support member comprises an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shifting of said pinion.
 4. Apparatus in accordance with claim 3, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positions of said arm member in which said pinion is drivingly coupled to one of said driven gears being disposed on opposite sides of the vertical position, and said rotating eccentric weight being effective to prevent lodging of said arm in intermediate positions wherein said pinion is out of engagement with both driven gears.
 5. Apparatus in accordance with claim 4, wherein said biasing means comprises magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means straddling said arm member and positioned to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.
 6. Apparatus in accordance with claim 1, and means coupled to said pinion for driving said pinion unidirectionally, and an idler gear drivingly engaged with one of said driven gears, said pinion being engaged with said idler gear when drivingly coupled to one of said driven gears.
 7. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion shiftable between respective positions wherein it is drivingly coupled to one or the other of said driven gears, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion to initate shifting of said pinion, means coupled to said pinion for driving said pinion unidirectionally, and an idler gear drivingly engaged with one of said driven gears, said pinion being drivingly engaged with said idler gear when drivingly coupled to said one of said driven gears, said support member comprising an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shift of said pinion.
 8. Apparatus in accordance with claim 7, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positionS of said arm member in which said pinion is drivingly coupled to one or the other of said driven gears being disposed on opposite sides of the vertical position.
 9. Apparatus in accordance with claim 8, and biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said driven gears, said biasing means comprising magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means straddling said arm member and position to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.
 10. Apparatus in accordance with claim 9, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces, said rotating eccentric weight means being driven by said means for driving said pinion.
 11. A self-reversing drive mechanism for a flexible web, comprising first spool means adapted to receive one end of said web, second spool means adapted to receive the other end of said web, and means for alternately driving said first and second spool means in opposite directions, said means for driving said first and second spool means comprising respective driven gears operatively associated with said first and second spool means, and a pinion, means for driving said pinion in a predetermined direction, said pinion being shiftable between respective positions wherein it is drivingly coupled to one or the other of said driven gears while said means drives said pinion in said direction, said pinion being rotatably mounted on a support member movable in response to radial tooth forces on said pinion while said pinion rotates in said direction to initiate shifting of said pinion, said support member comprising an elongated arm member rotatable about a pivot disposed adjacent a lower end thereof, said pinion being disposed at a location spaced from said pivot, whereby rotation of said arm about said pivot causes shifting of said pinion.
 12. Apparatus in accordance with claim 11, wherein rotation of said arm member about said pivot traverses a vertical position of said arm member, respective positions of said arm member in which said pinion is drivingly coupled to one or the other of said driven gears being disposed on opposite sides of the vertical position.
 13. Apparatus in accordance with claim 12, and biasing means for urging said support member to respective positions in which said pinion is drivingly coupled to one of said driven gears, said biasing means comprising magnetically attractable means coupled to said arm member remote from said pivot, and fixed magnetically attractable means straddling said arm member and position to contact said first-mentioned magnetically attractable means when said arm member is in said respective positions.
 14. Apparatus in accordance with claim 13, and rotating eccentric weight means coupled to said support member for effecting shifting of said support member after initiation of shifting by the tooth forces, said rotating eccentric weight means being driven by said means for driving said pinion. 